1. Field of the Invention
The present invention relates to telemetry equipment and methods, and particularly to sonic telemetry apparatus and methods used in the oil and gas industry associated with oil and gas exploration drilling, and in particular with the placement of the borehole in the earth in relatively precise alignment with a reference well or other reference locations.
2. Description of the Related Art
Ranging is the description of a general method whereby a specific measurement technique is used to determine the position of a borehole being drilled relative to a reference such as a surface reference or references or another borehole or set of boreholes. The position of the borehole being ranged specifically relates to the orientation, spacing or separation along all or part of the borehole relative to the reference.
In some drilling circumstances it may be important to determine the relative position of one or more boreholes in order to attain a certain separation and orientation (e.g. river crossing or steam-assisted gravity drainage (SAGD) well pair) or to either seek (e.g. relief well) or avoid (e.g. anti-collision) intersection between boreholes.
Avoiding intersection between boreholes that have been drilled when drilling a new borehole may be required on platforms or in areas that are congested with many previously drilled boreholes.
Achieving intersection between a borehole that has been drilled when drilling a new borehole may be required when a drilling a relief borehole or when linking a new borehole with an existing borehole.
Determining the relative position of a borehole relative to a surface reference in order to attain a certain separation and orientation is important when drilling underground passages such as those for cables or pipelines. These passages may be required to go under mountains, cities, roads, railroads or rivers or similar obstructions.
Determining the relative positions of two or more boreholes in order to attain a certain separation and orientation is important when a steam assisted gravity drainage production technique (SAGD) or similar techniques are used. The SAGD production technique for heavy oil, for example, involves the drilling of an upper and lower borehole pair with boreholes oriented in the same vertical plane and parallel to each other along the entire length of their horizontal sections. Steam that is injected into the upper borehole reduces the viscosity of the heavy hydrocarbons that are contained in the formations surrounding the upper borehole, thereby enabling these hydrocarbons to flow toward the lower borehole as a result of gravity. These hydrocarbons are then produced from the lower borehole using conventional production techniques. In order for the SAGD production technique to be successful, precise directional control must be maintained during the drilling of the borehole pair. Both the orientation and the distance between the boreholes must be precisely achieved. Typical separation distances between upper and lower boreholes are 15 m, with vertical and lateral relative displacements being held preferentially to within 1 m. This relatively precise directional control can be achieved using ranging techniques.
At present the ranging associated with SAGD wells is accomplished with certain magnetic techniques. The various shortcomings of commercial magnetic ranging techniques are discussed below.
The most recent, but so far commercially unproven, magnetic ranging technique requires that the casing in the target well be magnetised at various locations along the wellbore. A magnetic receiver system is installed in the well being ranged and ranging data are transmitted to surface using conventional MWD telemetry.
Existing commercial magnetic ranging techniques typically require the use of a wireline logging system to be deployed in the target well which can either be the injection or producing well. A tractor system is also required to convey the wireline magnetic receiver tool along the horizontal section of this wellbore. Alternatively, a coiled tubing system or a conventional jointed pipe rig can be used to convey the wireline magnetic receiver tool along the high angle sections of wellbore. In all cases the receiver tool must advance along the target wellbore in unison with the progress of the magnetic source in the well being drilled. Depth control must be achieved in both wells. Typical stated accuracy using magnetic ranging is about 100 cm. The present systems for drilling SAGD wells are particularly complex and would benefit in terms of reliability and cost with a simpler system.
The costs associated with the use of existing commercial magnetic ranging techniques include:                magnetic source tool        magnetic receiver tool        wireline telemetry system        wireline tractor for deploying wireline receiver tool or—        coiled tubing system for deploying wireline receiver tool or—        rig for deploying wireline receiver tool        surface ranging processing system        
The use of such equipment typically costs the producer several million dollars per well, thus showing that cost reduction would be a distinct benefit. Our invention uses a sonic ranging technique that is simpler and less expensive than magnetic ranging techniques, albeit with lesser ranging accuracy. We describe some of its benefits below. Our sonic ranging technique provides a simpler ranging solution in comparison to existing magnetic ranging systems due to the replacement of a tractor, rig or coil tube drilling (CTD) deployed wireline magnetic receiver with a surface deployed sonic transmitter. The sonic receiver is deployed in the bottom hole assembly (BHA) of the well being ranged and ranging data is either sent uphole via a measurement while drilling (MWD) system (also required for measurement of toolface orientation) or processed by a closed loop rotary steering tool (RST) system downhole.
Pertinent ranging techniques can be readily understood by reference to “Understanding GPS: Principles and Applications”, Artech House, 1996, edited by E. D. Kaplan, section 2.1.
The costs associated with the use of the sonic ranging technique include surface source(s) and a sonic downhole receiver and either a surface ranging processing system for ranging data sent uphole via MWD telemetry and a downhole “short hop” or “direct wire” interface between the sonic receiver and the MWD system (if not already in place with standard equipment), or a downhole RST and downhole “short hop” or “direct wire” interface between the sonic receiver and the RST system (if not already in place with standard equipment), thereby leading to a dramatic reduction in equipment cost.
Furthermore the use of a sonic ranging system and a RST allows full closed loop control to be executed downhole without the need for surface processing or control using a directional driller. The accuracy and precision of the sonic ranging system is approximately an order of magnitude less than the magnetic ranging system (which may be accurate to 100 cm). However, this level of accuracy and precision is still acceptable for useable placement of wells such as SAGD well pairs or similar at a significantly lower cost because wellbore placement accurate to ˜1 m is adequate in most instances. Indeed, this accuracy is also acceptable for the placement of single generally horizontal wells such as river crossing or road crossing etc. wells.